How do you stop a speeding starship?

The aim of the Starshot project is to send a tiny spacecraft propelled by an enormous rectangular photon sail to the Alpha Centauri star system, as shown in this artist’s conception, (Planetary Habitability Laboratory, University of Puerto Rico at Arecibo)

Millions of dollars are being spent on a scheme to speed up swarms of tiny sail-equipped probes to 20 percent of the speed of light and send them past Alpha Centauri – but how do you slow them down again?

German researchers suggest using the same light sails that got the probes going so fast in the first place.

The Starshot mission, backed by Russian billionaire Yuri Milner and other luminaries, calls for shooting nano-probes into space and having them unfurl large, lightweight sails. An array of antennas would beam photons at the sails. That radiation pressure would push the probes at a constantly rising velocity toward the Alpha Centauri system, a little more than 4 light-years away.

At an average speed of 20 percent of the speed of light, the nano-probes would zoom past the alien stars after 20 years of cruising. But their velocity wouldn’t leave much time for observations. If the probes kept going at peak velocity, they’d cover a span equivalent to the distance between Earth and the moon in just six seconds.

The researchers call for the probes’ light sails to be redeployed, facing the other way, so that the radiation pressure from Alpha Centauri’s suns would push against the probes as they approach. The steerability would come by positioning the sail at a specific angle, just as mariners steer their sailboats by moving the sail into the wind.

Computer simulations suggest that the trick should work, assuming that each probe weighs 100 grams and is mounted on a 100,000-square-meter sail, which is equivalent to the area of 15 football fields. The photon pressure would provide the initial slowdown, allowing the probes to be captured by the stars’ gravitational fields.

In order to put the probes into orbit around Alpha Centauri A, the system’s brightest star, they’d have to come within 2.5 million miles of the star.

They’d also have to be going slower than what’s called for in the Starshot mission plan. If a probe’s velocity exceeds 4.6 percent of the speed of light, it would overshoot the star. But the way Heller and Hippke see it, the payoff would be worth the extra travel time.

“In our nominal mission scenario, the probe would take a little less than 100 years – or about twice as long as the Voyager probes have now been traveling. And these machines from the 1970s are still operating,” Hippke said in a news release.

The researchers also have worked a trajectory that allow the probes to be accelerated toward Alpha Centauri by solar radiation rather than photon beams from Earth, and a complicated path that slingshots the probes around Alpha Centauri A and B to get them to Proxima Centauri.

They’re discussing their concept with Breakthrough Starshot team members.

“Our new mission concept could yield a high scientific return, but only the grandchildren of our grandchildren would receive it. Starshot, on the other hand, works on a timescale of decades and could be realized in one generation,” Heller said. “So we might have identified a long-term, follow-up concept for Starshot.”